314 
THE VOYAGE OF H.M.S. CHALLENGER. 
pumice. Finally, it may be pointed out that just as the lapilli of compact basic glasses 
are abundant in a pelagic deposit, so are the splinters of obsidian, properly so called, rare 
in the same soundings or dredgings. 
Volcanic Ashes . — The minuter glassy and mineral particles of a volcanic eruption are 
usually called volcanic ashes, and differ only from the lapilli, with which indeed they are 
often associated, by their smaller dimensions. They may, however, occur without these 
larger fragments, and they have as a general rule a much wider dispersion ; indeed their 
distribution in deep-sea deposits is as extensive as that of pumice. By casting the eye 
down the columns of mineral particles in the Tables of Chapter II., it may be seen that 
fragments which might be called volcanic ashes are much more widely distributed than 
the lapilli or fragments of rocks. This arises from the circumstance that these volcanic 
miueral particles may have a double origin : they may have been projected as isolated 
grains by subaerial or submarine eruptions, or they may be derived from the mechanical 
disintegration or the chemical decomposition of pumice or lapilli, of which they formerly 
were constituent parts. In other words, they may be volcanic ashes in the proper sense 
of the term, or the residue of pumice and of lapilli more or less destroyed. All that was 
.said at the commencement of this chapter with regard to the distribution of pumice 
applies equally to these incoherent particles. It will suffice to recall the phenomena 
witnessed in Iceland in 1874 and in Krakatoa in 1883, together with many similar 
instances, with reference to the projection of ashes, to understand the vast extent over 
which these particles may be spread. Granted the origin of these fragmental materials 
from the pulverisation of liquid lava, their transportation by air and by water, one would 
expect to find these volcanic dusts everywhere in the deep-sea deposits ; and remember- 
ing the rapidity with which they must have cooled, they should be present as vitreous 
particles, with the minerals which were ejected with them, in more or less embryonic 
form, and generally covered with a vitreous coating. This is, in fact, what is observed 
in pelagic deposits. 
It seems important here to recall what is understood by recent volcanic particles 
in opposition to mineral particles derived from the decomposition of ancient rocks of 
continentid origin, which make up the larger part of terrigenous deposits. The researches 
of the bust few years seem to have shown that the subdivision, before generally admitted, 
into Tertiary and Post-Tertiary volcanic rocks, and into Plutonic or Pre-Tertiary rocks, 
cannot now bo maintained. If we have, however, designated as recent rocks those 
s{x>ken of up to this j)oint in the present chapter, it is only because their position at the 
bottom of the sea and their lithological nature appear to require this classification. We 
seem to Ikj justified in regarding these volcanic ashes as recent volcanic products like the 
pumice and the lapilli, with which they are associated in deep-sea deposits. It becomes 
ver)^ difficult to make this distinction among the mineral particles, especially when 
the products of recent eruptions are mixed in the terrigenous muds with the debris of 
